JoinableMap.scala

implicit class JoinableMap[K, V1](left: Map[K, V1]) {
  def innerJoin[V2](right: Map[K, V2]): Map[K, (V1, V2)] = (
    for {
      k <- (left.keySet & right.keySet).toSeq
      l <- left.get(k)
      r <- right.get(k)
    } yield (k -> (l, r))
  ).toMap
  
  def leftJoin[V2](right: Map[K, V2]): Map[K, (V1, Option[V2])] = (
    for {
      k <- left.keySet
      l <- left.get(k)
    } yield (k -> (l, right.get(k)))
  ).toMap
  
  def rightJoin[V2](right: Map[K, V2]): Map[K, (Option[V1], V2)] = (
    for {
      k <- right.keySet
      r <- right.get(k)
    } yield (k -> (left.get(k), r))
  ).toMap
  
  def outerJoin[V2](right: Map[K, V2]): Map[K, (Option[V1], Option[V2])] = (
    for {
      k <- (left.keySet | right.keySet).toSeq
    } yield (k -> (left.get(k), right.get(k)))
  ).toMap

  @inline
  def join[V2](right: Map[K, V2]): Map[K, (V1, V2)] = innerJoin(right)
}